The present invention relates to an end fitting for a fuel assembly of a nuclear reactor comprising an elastically drawn back rigid lever.
The fuel assemblies of water-cooled nuclear reactors generally comprise rigid end fittings, namely a lower end fitting and an upper end fitting, fixed to each end of rigid guide tubes. The guide tubes carry a plurality of spacing grids, which maintain in place a group of fuel rods.
Assemblies of this type, when fitted into the reactor core, are positioned between an upper plate and a lower plate forming part of the structure placed in the reactor vessel. They are longitudinally swept from bottom to top by a cooling water stream. The difference between the thermal expansion coefficients of the materials used in the nuclear reactor structure prevents the rigid fixing of fuel assemblies to the upper and lower plates of the core structure.
Thus, it is conventional practice to place between each assembly and one of the core plates, generally the upper plate, an elastic fixing member, which forces back the assembly against the opposite plate.
However, the problem exists of the fracture of the elastic fixing member, because the broken part thereof can be carried along by the heat-transfer fluid and will lead to damage in the cooling water circuit. Solutions have already been proposed for eliminating this problem. For example, U.S. Pat. No. 4,072,562 granted Feb. 7, 1978 relates to a device having combined flexion and torsion springs mounted in the end fitting of the assembly. The spring is formed by a first flexion arm held in a cavity, a torsion arm and a second flexion arm exerting pressure on a projection of the grid plate.
The torsion arm which is most likely to break is covered with a protective metal sheet serving to prevent broken spring fragments from entering the primary cooling fluid circuit. For the same purpose, the second flexion arm is terminated by a material back-flow, which is displaced in a guidance slot.
Thus, in this device, the problem of the breaking of springs has been considered and a solution proposed for preventing a broken part of the spring from being carried along in the heat-transfer fluid, causing damage in the cooling water circuit. However, in spite of the fact that therefore at the part of the spring working in torsion and which is most likely to break has been covered with a protective sheet, another weak point exists at the bottom of the second flexion lever. This part, which has a constant section, works both in flexion and in torsion, whilst the bending stress is at a maximum level there, because it is at a maximum distance from the point by which the force of the spring is transmitted. Finally, it is curved, which further contributes to the weakening thereof.
Thus, a fracture can take place at the base of the second torsion arm, which will then be carried along by the cooling fluid and can even leave the slot as a result of its agitation in the cooling fluid. However, even if the broken part of the flexion arm remains in the slot as a result of the material backflow, its displacement may cause damage.